1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, in particular, to an improved CMOS comparator design.
2. Discussion of the Prior Art
Typical problems associated with any comparator design are speed, input referred offset voltage and noise, and power supply rejection.
A CMOS comparator which is representative of the prior art is disclosed by H. L. Fiedler and B. Hoefflinger, "A CMOS Pulse Density Modulator for High Resolution A/D Converters", 1984 IEEE.
The Fiedler/Hoefflinger circuit includes no input cascode devices, utilizes a double turn-around prior to the output, and provides no current mirrors before the latch stage. These characteristics contribute to power supply coupling and Miller feedback to the high impedance input. Both of these effects can add undesirable charge onto the high impedance capacitor which is holding the desired input signal.